Digital cellular telephone networks have relied in the main upon circuit switched channels to carry user traffic such as voice communications. A circuit switched channel is formed by the allocation of one slot per frame in a given TDMA channel. Whilst circuit switched connections have proved adequate for voice calls, they do not provide an efficient mechanism for transferring large amounts of data. For example, a user wishing to view a web page hosted on an Internet server will likely find that it takes an inordinately long time to download the page to a mobile telephone using a circuit switched connection.
To facilitate fast data transfers to mobile terminals, packet switched data services are being introduced to digital cellular telephone networks. For example, the General Packet Radio Service (GPRS) is currently being introduced to many GSM networks. Unlike circuit switched connections, a GPRS connection (referred to as a PDP context) for a given user does not necessarily occupy one slot per frame on a given TDMA channel. Rather, slots are only used when the user has data to send or receive. When there is no traffic to transmit, no slots are allocated to the user. When there is a large volume of data to transmit, the user may be allocated one or more slots per frame. GPRS will be available in future third generation networks such as UMTS networks. The introduction of packet switched services, and the resulting availability of high data transfer rates, will allow network operators to increase the range of services available to subscribers as well as to improve already available services. For example, multimedia services such as videoconferencing and document sharing are likely to prove extremely popular.
To facilitate the provision of multimedia services, the 3rd Generation Partnership project (3GPP) has been developing a so-called IP Multimedia Core Network Subsystem (IMSS). IMSS communicates with the GPRS core network and contains all elements that are used to provide IP based multimedia services. For a mobile to mobile call, the IMSS will sit between two GPRS core networks (assuming the mobiles belong to different networks). Certain nodes of the IMSS may be owned by the operator of a first of the GPRS networks, with the remaining nodes being owned by the operator of the second network (some IMSS nodes may be owned by a third party). The base protocol for multimedia services is the IETF Session Initiation Protocol (SIP). SIP makes it possible for a calling party to establish a connection to a called party even though the calling party does not know the current IP address of the called party, prior to initiating the call. SIP provides other functionality including the negotiation of connection parameters (e.g. Quality of Service and codecs).
The introduction of new services will provide operators with both opportunities and challenges when it comes to charging subscribers. Certain operators may decide to charge subscribers separately for using the GPRS access network (e.g. based on a charge per unit of data) and for using a service (e.g. based on a charge per unit time). However, it is more likely that operators will want to issue only a single charge for using a multimedia service, for example so as to be able to offer a lower total tariff which is less than the sum of the two separate tariffs. Charging information generated by the GPRS access network (at either a GGSN or a SGSN) will be identified using a GPRS charging identity, whilst charging information generated by the IMSS (at a P-CSCF or S-CSCF node) will be identified using a SIP call identity. It will therefore be necessary for operators to be able to marry or match charging information associated with a given call in order to be able to generate a single charge. This applies to conventional charging systems in which Call Detail Records (CDRs) are sent from charging nodes to a mediation device or billing system, as well as to real time charging mechanisms using for example, the CAMEL Application Part (CAP) protocol.